This invention relates to a method of, and apparatus for use in, magnetic resonance imaging; and more particularly, to contrast agent enhanced magnetic resonance arteriography for examining, detecting, diagnosing, and treating arterial diseases and injuries in arteries in the lower extremities, including defining anatomic features relevant to performing arterial surgery for atherosclerotic disease.
Arterial diseases and injuries are common and often have severe consequences including death. Imaging arteries serves to screen, detect and characterize arterial disease before these consequences occur. It also serves to define anatomic features which may provide assistance when performing surgery for atherosclerosis.
Atherosclerosis is a major problem in the aged population, particularly those in developed countries. This disease tends to be progressive in a considerable number of instances and may result in significant morbidity; and, in instances of severe atherosclerotic disease, lower limb amputations and/or mortality.
Early detection of atherosclerosis may lead to a decrease incidence of complications by allowing earlier treatment of arterial stenoses or atherosclerotic disease. Of the conventional techniques, catheter arteriography is the "gold standard" for delineation of the arterial tree of the lower extremities. This technique involves inserting a catheter into the artery of interest (the artery under study) and injecting radiographic contrast, for example, an iodinated contrast, while acquiring radiographs of the artery. Radiographs are commonly referred to as X-rays. The contrast remains in the arteries for a few seconds during which time the arteries appear distinct from both the veins and background tissue in the radiographs.
Although a catheter-based contrast arteriography technique generally provides high quality arterial images, there is a risk of arterial injury or damage by the catheter and its insertion. There may be thrombosis, dissection, embolization, perforation or other injury to the artery itself. Furthermore, such a technique may result in a stroke, loss of a limb, infarction or other injury to the tissue supplied by the artery. In addition, hemorrhage at the catheter insertion or perforation sites may require blood transfusions. Moreover, kidney failure and brain injury may result from the toxic effects of the X-ray contrast.
Because of its invasive nature, cost and complication rate, catheter arteriography is typically not a suitable screening technique for detection of stenoses or atherosclerotic disease. Rather, catheter arteriography is most often used prior to angioplasty or surgical reconstructive procedures--that is, after detection and diagnosis of a stenoses or atherosclerotic disease.
Further, although catheter arteriography is highly accurate under ideal conditions, it often fails to demonstrate distal vessels suitable for bypass in more than half of the patients with severe disease. Additionally, overlapping cortical bone can make interpretation of overlying vessels difficult.
There are several conventional non-invasive tests for diagnosis of peripheral vascular disease including B-mode and color flow Doppler sonography. Sonography provides indirect information of stenoses based on waveforms and velocity measurements. These type of tests require a skilled and experienced examiner to maintain acceptable accuracy and are often problematic in obese individuals. Although imaging from the groin vessels to the level of the mid popliteal artery is possible in many patients, arterial information below the mid popliteal artery is frequently inaccurate. In addition, because sonography provides indirect information of the arterial characteristics, sonography cannot image stenoses directly in the majority of cases. As such, estimation of the degree of luminal stenosis relies on velocity measurements which can often be inaccurate.
Magnetic resonance angiography has several advantages over conventional or catheter arteriography. Magnetic resonance angiography does not use ionizing radiation, and does not require arterial catheterization and sometimes can be performed without contrast agent enhancement. Even when performed using a contrast agent (e.g., gadolinium), magnetic resonance angiography contrast is safer than iodinated contrast arteriography, and it infrequently causes the patient discomfort. In addition, the cost of an magnetic resonance arteriogram is less than a catheter arteriogram. Further, because of its sensitivity to slow flow MRA may be more sensitive in vessels with proximal stenoses.
However, there are several impediments or limitations to use of magnetic resonance angiography as a satisfactory screening tool for imaging of the lower limb vasculature. These impediments or limitations have been documented in, for example, Owen, et al., Magnetic Resonance Imaging of Angiographically Occult Runoff Vessels in Peripheral Arterial Occlusive Disease, N Eng. J Med 1992, 326:157-1581; Owen et al., Symptomatic peripheral vascular disease: Selection of imaging parameters and clinical evaluation with MR angiography, Radiology 1993, 187:627-635; Yucel et al., Atherosclerotic occlusive disease of the lower extremity: prospective evaluation with two-dimensional time-of-flight MR angiography, Radiology 1993,187:635-641; and Borrello, MR Angiography versus conventional X-ray angiography in the lower extremities: Everyone wins, Radiology 1993,187:615-617).
One of the limitations to employing contrast enhanced magnetic resonance angiography for imaging of the lower limb vasculature has been the lack of a suitable coil for imaging of a sufficiently large field-of-view which encompasses the lower extremities. Another potentially more serious problem involves limitations of magnet imaging region. In this regard, the length of the magnet imaging region for conventional magnet resonance apparatus is insufficient to cover the entire anatomical region-of-interest in one acquisition. This can be as much as 120 centimeters in tall patients and even in small patients an imaging field encompassing approximately 90 centimeters is necessary to compete with arteriography which images from above the aortic bifurcation downwards.
Because of the reliance of traditional magnetic resonance arteriography methods on time-of-flight effects, imaging orthogonal to the plane of the vessel necessitates image acquisition in the axial plane. As such, it is necessary to reposition the patient and perform an additional localizer for successive locations down the leg--all of which is time-consuming. In order to maintain spatial resolution, thin slices must be obtained giving poor spatial "coverage" per unit time. Under these circumstances, imaging times may be in excess of one hour and frequently 2 hours for comprehensive imaging of the lower limb vessels. Although acquisition of the 2-D time-of-flight images in the coronal plane would significantly reduce imaging time and therefore cost, imaging in this plane would result in saturation of the flowing blood and non-diagnostic studies.
One solution to the problem of in-plane saturation has been to employ contrast enhanced magnetic resonance arteriography to overcome saturation effects. See, e.g., U.S. Pat. Nos. 5,417,213; 5,553,619; and 5,579,767 (the contents of each are hereby incorporated by reference). Using this technique, acquisition of the central lines of k-space which govern image contrast are acquired during peak arterial enhancement by carefully timing the injection of the contrast agent with the collection of image data which is representative of the center of k-space. A technique of more precisely acquiring the central lines of k-space during peak arterial enhancement by detecting arrival of the gadolinium bolus and initiating data acquisition of data which is representative of the central lines of k-space is described in U.S. Pat. No. 5,590,654. These technique have been shown to be highly accurate compared to arteriography or surgical inspection in the evaluation of abdominal aortic aneurysms, thoracic aorta, renal and mesenteric arteries, and aorta and iliac vessels.
In spite of the high quality images of the abdominal aortic aneurysms, thoracic aorta, renal and mesenteric arteries, and aorta and iliac vessels which have been consistently obtained using contrast enhanced magnetic resonance arteriography, there still remains several problems with using such imaging techniques to evaluate arteries in lower extremities. By using a gadolinium contrast agent, high signal-to-noise images are generated within the body coil. Although this overcomes the problems of both in-plane saturation effects and the necessity for expensive and as yet experimental surface coils, the imaging volume is limited by the largest field-of-view. The field of view, however, is governed by the physical dimensions of the body magnet or coil (typically 48 centimeters or less). Even if this large field-of-view could be used, the increased matrix size necessary to maintain resolution would increase examination time (increased number of phase-encoding steps) and increase echo time (increased frequency-encoding steps), both of which are undesirable. Additionally, the signal-to-noise from the top and bottom ends of the imaging volume would likely be inadequate for diagnostic purposes; and even if adequate images were obtained over the entire imaging volume, the anatomical coverage would still be insufficient for adequate evaluation of the lower extremities.
Although a second sequence centered at a lower level would provide the "missing" diagnostic information, this presents additional concerns of the time dependence of the arterial signal which would likely diminish to a level below that necessary for diagnostic imaging in relation to the time necessary for relocalization and re-prescription of another sequence at another, lower level. This time delay in collecting image data for the second, lower level would be of such a magnitude that enhancement of lower limb veins would complicate interpretation of images and probably render lower extremity arterial imaging non-diagnostic.
As a result, there exists a need for an improved apparatus and method for magnetic resonance arteriography which provides an image of the arteries distinct from the veins and which overcomes the limitations of other techniques. There exists a need for an apparatus and technique which allows preferential imaging of the lower limb arterial tree in a sufficiently short time period to allow imaging without significant venous overlap and without the complications often observed or experienced with catheter arteriography.
In addition, there exists a need for a contrast (e.g., gadolinium) enhanced magnetic resonance arteriography technique which provides essential and accurate anatomic information for arterial reconstructive surgery and which is devoid of contrast-related renal toxicity or catheterization-related complications attending catheter arteriography.